A sump pump has to do two things: move water out faster than it flows into the pit, and push it up and out through your discharge pipe. This calculator works out both numbers, the flow in gallons per hour (GPH) and the total dynamic head in feet, using Zoeller’s published sizing method. Once you have both numbers, our sump pump brands guide shows which makers publish flow at a stated head.
This calculator is for estimating only — have a licensed plumber confirm sizing. Wiring, discharge routing and local code are part of the final choice.
How to size a sump pump: you need two numbers. For flow, time how many inches the water in the pit rises during heavy inflow. The pit’s area in square inches ÷ 231 is gallons per inch; multiply by the inches of rise and divide by the minutes timed to get GPM (× 60 for GPH). For head, add the vertical lift to the friction head (from Zoeller’s pipe and fitting tables) and round up to the next foot. Pick a pump whose chart shows more than that flow at that head.
How the calculator works
Gallons per inch of pit = the pit’s floor area in square inches ÷ 231, because a U.S. gallon is 231 cubic inches (NIST). A round pit’s area is 3.1416 × (diameter ÷ 2)². An 18-inch round pit holds about 1.1 gallons per inch; a 24-inch pit about 1.96.
Inflow (GPM) = gallons per inch × inches of rise ÷ minutes timed. Multiply by 60 for GPH.
Total dynamic head (feet) = vertical lift + friction head (Zoeller). Friction head = (the pipe’s actual length, vertical lift plus horizontal run, + the equivalent length of the fittings) × Zoeller’s friction loss per 100 feet at your flow ÷ 100. The calculator uses the first row of Zoeller’s table at or above your flow, then rounds the total up to the next whole foot, as Zoeller’s example does.
| Pipe | 90° elbow | 45° elbow | Check valve |
|---|---|---|---|
| 1-1/4 inch | 3.5 | 1.8 | 11.5 |
| 1-1/2 inch | 4 | 2.2 | 13.4 |
| 2 inch | 5.2 | 2.8 | 17.2 |
Zoeller’s own worked example: 100 feet of 1-1/2 inch pipe plus 21.4 feet of fittings at 6 GPM is 121.4 × 0.33 ÷ 100 = about 0.4 feet of friction head. With 13 feet of lift, total dynamic head is 13.4 feet, rounded up to 14.
Worked examples
| Pit and timed rise | Inflow | Discharge pipe | Friction head | Look for |
|---|---|---|---|---|
| 18-inch round pit, rises 3 in. in 60 s | 3.3 GPM (199 GPH) | 8 ft lift, 10 ft run | 0.13 ft* | More than 199 GPH at 9 ft |
| 18-inch round pit, rises 6 in. in 60 s | 6.6 GPM (397 GPH) | 10 ft lift, 20 ft run | 0.29 ft | More than 397 GPH at 11 ft |
| 24-inch round pit, rises 6 in. in 60 s | 11.8 GPM (706 GPH) | 10 ft lift, 25 ft run | 0.65 ft | More than 706 GPH at 11 ft |
| 24-inch round pit, rises 10 in. in 45 s | 26.1 GPM (1,567 GPH) | 12 ft lift, 40 ft run | 4.6 ft | More than 1,567 GPH at 17 ft |
*Zoeller’s 1-1/2 inch rows start at 6 GPM, so a smaller inflow uses the 6 GPM row (0.33 ft per 100 ft). That errs a little high. Total dynamic head is the lift plus the friction head, rounded up to the next foot, as in Zoeller’s example.
Limits of this estimate
- One timed fill is one sample. Test during the heaviest inflow you see, because a dry-season test will undersize the pump.
- Zoeller’s friction table covers 1-1/4, 1-1/2 and 2 inch pipe up to 35 GPM (2,100 GPH). Above that, have a pro size it.
- Performance charts differ from pump to pump, and horsepower alone does not tell you flow at your head. Zoeller calls 1/3 HP the standard size for most homes but says to check your own numbers.
- It does not cover wiring, the discharge outlet location or local code, which a licensed plumber should confirm.
FAQ
How many GPH should my sump pump be?
More than the rate water flows into your pit, at your total dynamic head. Zoeller says to measure how much water the basin collects during a high-flow period and choose a pump that removes it faster. A timed fill test like the one above gives that number.
What is total dynamic head?
Zoeller defines it as static head (the vertical lift) plus friction head (the loss from pipe length, pipe size and fittings such as elbows and check valves). It is measured in feet, and you read it against a pump’s performance chart.
Does a battery backup pump need the same sizing?
It pumps into the same pipe, so check it against the same total dynamic head. Compare a backup pump’s rated flow at that head, or its maximum lift, on its listing before you rely on it.
Related guides
Sources
- Zoeller, “5 Steps to Sump Pump Sizing” (Sept. 15, 2023)
- NIST, “How Do You Measure Fuel at the Gas Pump?” (the U.S. gallon is 231 cubic inches)
Sources checked September 24, 2026.